Method for the single-sided polishing of bare semiconductor wafers

ABSTRACT

Single-sided polishing of bare semiconductor wafers is accomplished by using a polishing head with a membrane made of a resilient material by which polishing pressure is transmitted onto the backside of the semiconductor wafer to be polished, wherein the semiconductor wafer is pressed against a polishing cloth with a smooth surface while supplying a polishing agent, and is prevented from sliding off the membrane by a retainer ring. The retainer ring is provided with channels on a side surface facing the polishing cloth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the single-sided polishing (CMP, chemical-mechanical polishing) of bare (nonstructured) semiconductor wafers by using a polishing head with a membrane made of a resilient material, by which the polishing pressure is transmitted onto the backside of the semiconductor wafer to be polished.

2. Background Art

Polishing heads (or carrier heads) with a membrane (membrane carrier) are used in particular to planarize structures of electronic components. Occasionally, there are however also reports of their use for the polishing of bare semiconductor wafers. An example of this may be found in US 2002/0077039 A. It is a central aim of CMP to achieve both maximally high global and local planarity of the polished semiconductor wafer.

The polishing cloths are often provided with a surface structure (texture) formed by grooves. The grooves promote uniform distribution of the polishing agent on the polishing cloth and therefore also uniform polishing of the semiconductor wafers. US 2005/0202761 A1 describes a CMP method that uses a polishing cloth provided with grooves, which is optimized with respect to the distribution and the consumption of the polishing agent.

The requirements for the properties of bare semiconductor wafers with respect to local planarity are increasing constantly, especially in the nanotopography wavelength spectrum, and special efforts are necessary in order to be able to satisfy these requirements. Experiments by the present inventors have shown that the result of using membrane polishing heads to polish bare semiconductor wafers, in combination with textured polishing cloths, is that the nanotopography of the polished semiconductor wafers does not satisfy the requirements expected of polished wafers.

SUMMARY OF THE INVENTION

It was therefore an object of the present invention to overcome the previously described disadvantages, and to provide a method for the single-sided polishing of a bare semiconductor wafer by using a polishing head with a membrane made of a resilient material, which complies in full scope with modern requirements. These and other objects are achieved by a method for the single-sided polishing of a bare semiconductor wafer by using a polishing head with a membrane made of a resilient material, by which the polishing pressure is transmitted onto the backside of the semiconductor wafer to be polished, wherein the semiconductor wafer is pressed against a polishing cloth with a smooth surface while supplying a polishing agent and is prevented from sliding off the membrane by a retainer ring, and wherein the retainer ring is provided with channels on a side surface facing the polishing cloth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the nanotopography of a bare wafer polished in accordance with the present invention; and

FIG. 2 illustrates the nanotopography of a wafer polished with a textured cloth as used in the prior art to planarize wafers structured with electronic device features.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The inventors have unexpectedly discovered that unfavorable nanotopography is essentially attributable to the surface structure of the polishing cloth being used. The polishing cloth was provided with a pattern of grooves, in order to achieve a surface-wide supply of polishing agent to the polishing cloth and to facilitate lifting of the semiconductor wafer from the polishing cloth after polishing.

The claimed method does not use such polishing cloths, in order that the requisite nanotopography can be achieved. Instead, the semiconductor wafer is polished on a polishing cloth without texture, i.e. with a smooth surface, a smooth surface being intended to mean a surface which has no artificially added indentations, for example grooves or recesses, and no artificially added elevations, for example ridges or bumps. The surface of the polishing cloth needs to be smooth only in those regions which come in contact with the semiconductor wafer during the polishing.

The use of a polishing cloth with a smooth surface, however, also entails a problem which could be avoided by using a textured polishing cloth. The grooves in the polishing cloth facilitate lifting of the semiconductor wafer from the polishing cloth after polishing. The polishing agent contained between the polishing cloth and the semiconductor wafer ensures strong adhesion of the semiconductor wafer to the polishing cloth. The membrane of the polishing head is comparatively soft, so that the semiconductor wafer is susceptible to tilting and being separated from the membrane when the polishing head is lifted from a smooth polishing cloth. When attempting to lift the wafer with the polishing head from the polishing cloth, the semiconductor wafer may therefore remain on the polishing cloth. In order to prevent this, in the method according to the invention the polishing head is lifted from the polishing cloth after the polishing with a speed which is preferably no slower than 30 mm/min and no faster than 50 mm/min. With higher speeds, it often happens that the semiconductor wafer is left behind on the polishing cloth.

It is furthermore preferable to guide the polishing head by means of a groove in the polishing cloth or by means of the edge of the polishing plate after the polishing and before lifting the polishing cloth off. This likewise serves to reduce the adhesion of the semiconductor wafer. The groove lies in a region of the polishing cloth which is not covered by the semiconductor wafer during the polishing, preferably in an edge region, because according to the invention a smooth polishing cloth is required for polishing.

Another quality parameter, which must be considered for the polishing of bare semiconductor wafers, is the haze (microroughness). It has been found that particularly low haze values are obtained when a retainer ring, which is provided with channels on a side surface facing the polishing cloth, is used for the polishing. A suitable retainer ring is described, for example in U.S. Pat. No. 6,224,472 B1, which is incorporated herein by reference. For polishing bare semiconductor wafers with a diameter of 300 mm, the number of channels is preferably at least 30, more preferably at least 45, because the haze (microroughness) tends to decrease with an increasing number of channels.

Bare semiconductor wafers must be protected against contamination due to metallic impurities or particles. The membrane of the polishing head, which is in direct contact with the semiconductor wafer during the polishing, must therefore consist of a suitable material. It should as far as possible release no metals and have the lowest possible coefficient of friction, so that as few particles as possible are formed. Membranes made of silicone have proven to be particularly suitable.

The success of the invention will be demonstrated below by comparing one embodiment of the subject invention example with a comparative example.

EXAMPLE AND COMPARATIVE EXAMPLE

Semiconductor wafers made of silicon with a diameter of 300 mm were subjected to single-sided polishing, and the polishing result was examined in respect of the nanotopography. The groups of semiconductor wafers which were polished by the method according to the invention exhibited superior nanotopography after polishing, even though they were polished under the same conditions as the semiconductor wafers of the comparative example. The only difference was that the semiconductor wafers of the comparative example were polished on a textured polishing cloth. FIGS. 1 and 2 show the result of the nanotopography measurement on a semiconductor wafer of the example and a semiconductor wafer of the comparative example, respectively. Traces of the texture of the polishing cloth may be seen clearly (FIG. 2) in the case of the semiconductor wafer of the comparative example, which means that the nanotopography has been impaired.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 

1. A method for the single-sided polishing of a bare semiconductor wafer comprising polishing with a polishing head with a membrane made of resilient material by which polishing pressure is transmitted onto the backside of the semiconductor wafer to be polished, wherein the front side of the semiconductor wafer is pressed against a polishing cloth with a smooth surface while supplying a polishing agent, and is prevented from sliding off the membrane by a retainer ring, wherein the retainer ring is provided with channels on a side surface thereof, facing the polishing cloth.
 2. The method of claim 1, wherein the retainer ring has at least 30 channels.
 3. The method of claim 1, wherein the semiconductor wafer is guided by means of a groove in the polishing cloth or the edge of the polishing cloth after the polishing and before lifting the polishing head from the polishing cloth, wherein the groove is not located on a portion of the polishing cloth which will contact the wafer surface during polishing.
 4. The method of claim 2, wherein the semiconductor wafer is guided by means of a groove in the polishing cloth or the edge of the polishing cloth after the polishing and before lifting the polishing head from the polishing cloth, wherein the groove is not located on a portion of the polishing cloth which will contact the wafer surface during polishing.
 5. The method of claim 1, wherein the polishing head is lifted from the polishing cloth after polishing with a speed which is no faster than 50 mm/min.
 6. The method of claim 2, wherein the polishing head is lifted from the polishing cloth after polishing with a speed which is no faster than 50 mm/min.
 7. The method of claim 3, wherein the polishing head is lifted from the polishing cloth after polishing with a speed which is no faster than 50 mm/min.
 8. The method of claim 4, wherein the polishing head is lifted from the polishing cloth after polishing with a speed which is no faster than 50 mm/min.
 9. The method of claim 1, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 10. The method of claim 2, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 11. The method of claim 3, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 12. The method of claim 4, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 13. The method of claim 5, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 14. The method of claim 6, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 15. The method of claim 7, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone.
 16. The method of claim 8, wherein the semiconductor wafer is polished with the aid of a membrane comprising silicone. 